The present invention relates to an apparatus for forming a thin film consisting of a metal nitride or a metal oxide on a substrate that is to be treated.
According to the prior art, only narrow regions on the surface of the vaporizable substance could be locally irradiated with the electron beam generated by the hollow cathodic discharge, but wide areas on the surface of the vaporizable substance could not be scanned with the deflected electron beam.
To obtain a thin nitride film such as of silicon nitride or aluminum nitride, or a thin oxide film, such as of silicon oxide or aluminum oxide, a nitrogen gas or an oxygen gas is introduced into a vacuum vessel.
In this case, the vaporizable substance, such as silicon or aluminum heated at high temperatures in a crucible, is exposed to a nitrogen atmosphere or an oxygen atmosphere, and is nitrogenated or is oxidized to a considerable degree- The vaporizable substance exhibits a high melting point when it is nitrogenated or oxidized. Therefore, the vaporizable substance is not easily melted, and only a narrow area directly irradiated with the electron beam is heated at particularly high temperatures and melts and vaporizes. Accordingly, the vaporizable substance is not uniformly melted, and a hole is formed in a portion of the vaporizable substance by the electron beam. In other words, the vaporizable substance assumes the condition like in the crucible of the prior art forming a nitride layer 20 and a locally molten portion as shown in a section view of FIG. 14. Under such a condition, the vaporizable substance in the crucible is vaporized only partly whereby the rate of vaporization becomes unstable and the deposition time becomes short making it difficult to form the film up to a desired thickness.
Moreover, the electron beam forms a hole in the vaporizable substance so deeply that the crucible is often damaged.
According to the prior art in which a narrow area on the surface of a vaporizable substance is locally irradiated with an electron beam generated by the hollow cathodic discharge, the aforementioned problem takes place easily. The spot size of electron beam becomes small because of the pinch effect by the magnet field established by the electron beam itself. In order to magnify the spot size of electron beam, the magnetic field must be arranged and must have some strength. But it is difficult to provide a coil which established the magnetic field because a discharge current is as great as from several tens to several hundreds of amperes and the distance between the hollow cathode and crucible is several centimeters.
Moreover, when a voltage of a high frequency is applied as a bias to the substrate that is to be treated according to the prior art, the rate of vaporization of a vaporizable substance becomes unstable. Therefore, the density of ions is not stabilized in the atmosphere, the high-frequency impedance is not matched well, and a matching device must be frequently adjusted during processing.
FIG. 15 is a diagram showing an electron beam that is deflected by a magnetic field in an apparatus according to prior art.
A discharge current produced by the hollow cathodic discharge is so great as aforementioned that the electron beam 1 is deflected by an intense magnetic field 33 established by the discharge current. The deflected electron beam can hardly by focused on the center of the crucible 12. Therefore, the vaporizable substance 7 in the crucible 12 is not uniformly melted or is not vaporized for a long period of time, and the crucible 12 may be damaged by the deflected electron beam 1.